An image reader which reads a printed image and converts it to an electric signal is known. Such a device is provided with a contrast adjusting function with which the user may freely control contrast to obtain the optimum picture according to the original. Thus, as illustrated in FIG. 6, color signals R, G and B from an image sensor 1 are amplified by amplifying circuits 2, 3 and 4 and sequentially switched by a selection circuit 11 which is supplied with a switching signal from a device not shown.
Referring to FIG. 7, which is a specific embodiment of FIG. 6, the image sensor 1 outputs color signals R, G and B which are then independently amplified by amplifiers 2 to 4.
Taking the R signal as an example, FIG. 10(a) shows the waveform of the output signal from the amplifier 2 which is obtained when the picture elements of the image sensor 1 are sequentially scanned. In the image sensor 1, the plural picture elements in the first scanning area and those in the last scanning area constitute unused areas which are not used in the reading of a document. Thus, the areas T1 and T3 in FIG. 10(a) represent unused picture elements and the area T2, exclusive of these areas, represents effective picture elements exposed to light. As seen from FIG. 10(a), the output voltage of the area not exposed to light is a dark voltage, which is lower than the output voltage of the effective picture elements. Representative of this dark voltage, a predetermined value, for example a value found by averaging, is memorized by a sample hold circuit 5 in response to a timing signal from a circuit not shown, and the signal shown in FIG. 10(b) is output. The output signal of the sample hold circuit 5 and the output signal of the amplifier 2 are differentially amplified in a different amplification circuit 8. As a result, a signal available on subtraction of the dark voltage from the R signal is finally output as shown in FIG. 10(c).
The same applies to G and B signals, as shown in FIG. 7. Thus, the dark voltages of the respective color signals are memorized in sample hold circuits 6, 7, respectively, and subtracted from the color signals in differential amplification circuits 9, 10. The differential amplification circuit 8 comprises a differential amplifier 8a and resistors 8b through 8e, differential amplification circuit 9 comprises a differential amplifier 9a and resistors 9b through 9e, and differential amplification circuits 10 comprises a differential amplifier 10a and resistors 10b through 10e.
The output signals from the differential amplification circuits 8 through 10 are fed to a selection circuit 11, from which they are output in synchronism with selection signals supplied from a circuit not shown and converted to digital signals by and A/D converter 17.
The analog signal output of the selection circuit 11 is amplified by an amplifying circuit 12 with a gain predetermined according to a variable resistor 13 and converted to a digital signal in bits from a most significant bit (MSB) to a least significant bit (LSB) by the A/D converter 17. This digital signal corresponds to the levels of reference voltages H and L from a reference voltage generator 14 and amplifiers 15 and 16.
The contrast of the digital signal is determined by the variable resistor 13 and is increased as the gain of the amplifier 12 is increased. FIG. 8 shows the waveforms at respective stages in the circuit illustrated in FIG. 6. The illustrated example represents the case in which a red document is read. Here, only the R signal component of FIG. 8(a) is generated, with the G signal component of FIG. 8(b) and B signal component of FIG. 8(c) being absent. FIG. 8(d) shows the output signals of the selection circuit 11, indicating the sequential selection of the R, G and B signals, but only the R signal is output. FIGS. 8(e) and (f) show signals after gain adjustment. FIG. 8(e) represents the waveform pattern with contrast reduced by decreasing the amplification factor and FIG. 8(f) represents the pattern with contrast enhanced by increasing the amplification factor.
The above conventional image reader, in which the amplifier 12 deals with picture signals of large amplitude, is disadvantageous in that it requires a large throughput rate and is, therefore, expensive, and unless this requirement of large throughput rate is satisfied, the waveform is blunted as shown in FIG. 8(f), with the result that high fidelity color reproduction cannot be accomplished. Moreover, a variable resistor for contrast adjustment must be disposed at the periphery of a casing as shown in FIG. 9 so that the user may easily have access to it. This entails an undue routing of the signal as indicated by the bold line and, hence, degradation of picture quality. Conversely if priority is given to picture quality, the variable resistor will be limited in accessibility and exterior design.